The invention pertains to dual band power amplifiers such as those commonly employed in dual mode digital cellular telephones. More particularly, the invention pertains to a method, apparatus and technique for improving isolation between two physically adjacent amplifiers operating at different frequencies.
In cellular telephones and other electronic devices, it is often necessary for two power amplifiers operating on signals of different frequencies to be positioned very close to one another. For example, a dual mode, dual band digital cellular telephone is an example of one such component. Particularly, in the U.S., Europe, Japan, and other countries, the governments have assigned at least two frequency bands for digital cellular telephone communications. In the U.S. for instance, a first band, called the American Mobile Phone System or AMPS band, has been assigned at 824-849 MHz and a second band, called the Personal Communications System or PCS band has been assigned at 1850-1910 MHz. In Europe on the other hand, a first band called the Global System for Mobile Communications or GSM band has been assigned at 880-915 MHz and a second band, called the Digital Communications System or DCS band has been assigned at 1710-1785 MHz. Accordingly, dual band digital cellular telephones typically have two power amplifiers, one for each of the two bands. Note that, in both the U.S. and Europe, the higher frequency band is at about twice the frequency of the lower frequency band. Thus, in both the U.S. and Europe, the second harmonic of the lower frequency band is very close in frequency to the fundamental frequency of the higher frequency band.
In accordance with the ever-present desire to miniaturize most electronic devices, including cellular telephones, there is pressure to place the two power amplifiers physically very close to one another. When the conductors carrying the output signals of two amplifies are placed near each other, the output signals tend to magnetically couple to each other. If the frequency of the output signals of the higher frequency (high band) power amplifier is near a harmonic of the frequency of the output signals of the lower frequency (low band) power amplifier, spurious signals at the second harmonic of the lower frequency band that are generated at the output of the low band power amplifier that couple into the output of the high band power amplifier can be a problem. Specifically, these spurious signals that couple into the output conductors of the high band amplifier will bleed right through the filters and/or duplexers downstream of the high band power amplifier, reach the antenna, and be transmitted. Such high power spurious transmissions are, of course, undesirable. In fact, there are regulations regarding the maximum allowable spurious emissions for cellular telephones and other wireless devices. Further, the fourth harmonic of the low band power amplifier will be very close in frequency to the second harmonic of the high band power amplifier, which can also be a problem.
Several techniques have been tried to improve isolation of the two power amplifier bands within cellular telephones and other electronic devices. Probably the simplest technique is to provide as much physical separation between the two power amplifiers as possible. However, as noted above, cellular telephones, and particularly their electronic components, have become so small and the pressure remains to make them even smaller that increasing physical separation generally is not an acceptable solution. Another technique employed to maximize low band to high band power amplifier isolation involves using a second harmonic trap on the output of the lower band power amplifier to cancel second harmonic signals at the output of the low band power amplifier. It has been found, however, that there still tends to be quite a bit of second harmonic signal from the low band power amplifier coupling into the higher band power amplifier even with a second harmonic trap.
Another technique that has been used to improve isolation involves making the output conductors of one of the power amplifiers orthogonal to the output conductors of the other power amplifier. Thus, the magnetic field lines of the output signals of the low band power amplifier are orthogonal to the output wire bonds of the high band power amplifier so that they do not couple to each other as readily. However; this solution has some practical problems. Particularly, the power amplifiers are typically mounted on a printed circuit board or other substrate containing other RF driver and power control circuitry to which the power amplifiers must be coupled electrically. For instance, the outputs of the power amplifiers typically must be coupled to separate filters and/or duplexers or a diversity switch and then to the same antenna. Accordingly, it is often desirable for the input and output conductors of the two amplifiers to be parallel and/or adjacent to each other to shorten and/or simplify the conductor traces that must be placed on the printed circuit boards in order to carry the output signals to their ultimate destination, i.e., the same antenna. Making the output conductors of the two amplifiers orthogonal to each other can significantly complicate the design and layout realization of conductor traces on the substrate and increases the necessary size of the board.
Typical dual band power amplifier circuitry for cellular telephones and the like commonly are available in LTCC form, FR-laminate module form, or a large plastic molded package. Dual band power amplifier circuits available on the market today typically yield isolation power levels of about xe2x88x9225 dBm to xe2x88x9220 dBm. It would be desirable to attain better isolation.
Accordingly, it is an objective present invention to provide an improved dual band power amplifier.
It is another objective present invention to provide a dual band power amplifier with improved isolation between the two amplifiers.
The invention is a dual band power amplifier with a small footprint and having excellent band-to-band isolation. There are several aspects of the present invention that, acting together, maximize low-band-to-high-band isolation. The first and primary aspect is a dual frequency (second and fourth harmonic) trap at the output of the low band power amplifier comprising a first series capacitance, preferably, on the power amplifier chip, in series with an inductance, the inductance preferably in the form of a transmission line of predetermined length, and further including a second capacitance coupled between an intermediate point of the transmission line inductance and ground. This LC circuit forms a dual resonant second and fourth harmonic trap that provides a very low impedance at the output of the power amplifier at the second and fourth harmonics of the low band power amplifier. This type of harmonic trap substantially increases the band-to-band isolation comparative to a conventional second harmonic trap.
Band-to-band isolation can be additively increased by further forming a ground loop between the output conductors of the two power amplifiers. The ground loop is formed by using a looping wirebond that spans from one grounded location to another location positioned between the parallel output wirebonds of the high and low band power amplifiers. The ground loop further isolates the high band amplifier from the low band amplifier by causing the magnetic fields generated around the output wire bonds of the low band power amplifier to set up circulating currents primarily in the ground loop, rather than coupling into the output wire bonds of the high band power amplifier.
The invention finds particular use in cellular telephones for CDMA, TDMA and GSM. systems.